Electron Spin Resonance of Magnetic Two-Dimensional Covalent Organic Frameworks

Two-dimensional (2D) materials have generated significant interest because of their unique electrical, optical, and magnetic single-layer behaviors. However, the inability to add electrical or magnetic dopants to a substantial set of 2D materials hinders their ability to be incorporated into device architectures. Here, we synthesize 2D covalent organic frameworks (COFs) that have a lattice of nanopores, which we are able to synthetically fill with magnetic ions (Mn²⁺) that are hexagonally arranged. We use a host of characterization techniques, such as x-ray diffraction, TEM, NMR, and FTIR, to demonstrate that our COFs are ordered, nanoporous, and 2D. Magnetic ion incorporation is empirically shown through electron spin resonance measurements. Unlike the unfilled COFs, which have a single peak at g=2.0, the Mn-filled COFs show a hyperfine-split, sextet of peaks with a spin relaxation time of ~5 ns and a 19 G exchange splitting. The ability to chemically change the nanopore spacing and the inter-ion distance, combined with our confirmation of the hexagonal Mn²⁺ ion arrangement, suggests the possibility of using these 2D COFs for potential quantum spin liquids or in magneto-optical devices.